Defects in the regulation of innate immune responses are implicated in autoimmune diseases, including Systemic Lupus Erythematosus (SLE). During innate immune responses, the stimulation of B cells through Toll Like Receptors (TLRs) promotes polyclonal immunoglobulin secretion while simultaneously maintaining tolerance of autoreactive B cells. In autoimmune-prone individuals activation of innate immune responses fails to maintain B cell tolerance. Recently, we described that IL-6 and sCD40L secreted by dendritic cells (DCs) and macrophages (M&s) regulates autoreactive B cells during innate immune responses. DC/M&-mediated tolerance is selective in that only autoreactive, and not naove, B cells are repressed by IL-6 and sCD40L. In this application we propose to define the molecular basis for the selective repression of Ig secretion by DCs and M&s, and to test the hypothesis that defects in DC/M&-mediated tolerance contribute to the autoimmune phenotype of lupus-prone mice. The finding that IL-6R ligation selectively represses LPS-induced Ig secretion by autoreactive B cells suggests that chronic BCR stimulation reprograms the IL-6R, thereby regulating innate immune responses. In aim 1, we propose to define the mechanism(s) whereby IL-6 differentially regulates LPS-induced Ig secretion in autoreactive, compared to naove B cells. The findings that multiple repressive factors regulate Ig secretion suggested that IL-6 and sCD40L could exhibit redundant, or possibly non-overlapping functions. In subaim 2a, we will identify the B cell subset(s) repressed by DCs (IL-6) and M&s (IL-6+sCD40L), and determine whether dyregulation of DC/M&-mediated tolerance facilitates autoantibody secretion by select B cell subsets within autoimmune mice. Apoptotic cells are implicated in SLE and may activate autoreactive B cells. In subaim 2b, we propose to assess whether apoptotic cells diminish secretion of IL-6 and sCD40L by DC and M&-IL-6, thereby indirectly activating autoreactive B cells. In addition, we will determine whether apoptotic cells disrupt IL-6R reprogramming in B cells allowing autoreactive cells to become activated. DCs and M&s from lupus-prone mice are defective in repressing autoantibody production in vitro, although it remains unclear whether these defects cause the autoimmune phenotype in vivo. In subaim 3a, we propose to generate mixed bone marrow chimeras to address if reconstitution of MRL/lpr mice by wildtype DCs and M&s prevents the onset of autoimmunity or restores tolerance. To investigate if the loss of IL-6 and CD40L induce autoimmunity in non-autoimmune mice, we propose in aim 3b to monitor autoantibody production in mice where DCs and Ms are deficient in CD40L and/or IL-6.NARRATIVE Understanding the mechanisms that regulate autoantibody secretion is central to preventing autoimmunity and autoimmune-associated diseases such as systemic lupus erythematosus (SLE). This application characterizes molecular and cellular aspects of a novel tolerance mechanism wherein dendritic cells and macrophages regulate autoantibody secretion during bacterial and viral infections. We also propose to address the therapeutic potential of stem cell therapy as a treatment for SLE.